Pixel based machine for patterned wafers

ABSTRACT

A method is provided for the detection of defects on a semiconductor wafer by checking individual pixels on the wafer, collecting the signature of each pixel, defined by the way in which it responds to the light of a scanning beam, and determining whether the signature is that of a faultless pixel or of a pixel that is defective or suspect to be defective. An apparatus is also provided for the determination of such defects, which comprises a stage for supporting a wafer, a laser source generating a beam that is directed onto the wafer, collecting optics and photoelectric sensors for collecting the laser light scattered by the wafer in a number of directions and generating corresponding analog signals, an A/D converter deriving from the signals digital components defining pixel signatures, and selection systems for identifying the signatures of suspect pixels and verifying whether the suspect pixels are indeed defective.

FIELD OF THE INVENTION

The invention relates to the inspection of surfaces, particularly thesurfaces of semiconductor wafers, intended for the detection of possibledefects, particularly due to the presence of particles. Moreparticularly the invention relates to the control of semiconductormanufacturing processes, particularly Quality Control, ProcessMonitoring and Control and Catastrophe Detection. The invention furthercomprises method and apparatus for the inline control of waferproduction and the immediate recognition of any fault or irregularitiesin the production line.

BACKGROUND OF THE INVENTION

The detection of defects and/or of the presence of foreign substances onsemiconductor wafers has received considerable attention in the art.Defects can be caused by an imperfect production of the desired pattern.Further, particles of various kinds may adhere to a wafer surface for anumber of reasons.

The inspection process can be carried out on bare wafers, viz. wafersthat have not yet been patterned, or on patterned wafers. This inventionrelates primarily to the inspection of patterned wafers.

Prior art devices are used in order to detect defects and particles ofthe type described above in patterned wafers. Examples of prior artapparatus comprise devices based on the direct comparison of differentdies. Such apparatus, which will be further referred to below withrespect to specific references, presents the following drawbacks: 1) itis relatively very expensive, as it requires high mechanical precision;2) it has low throughput; 3) it has a large footprint; 4) it needs anexpert operator; 5) it is not suitable for inline inspection (i.e., itoperates on wafers which have been previously removed from thefabrication line), and therefore is unsuitable for purposes of processcontrol and monitoring, of the kind addressed by the present invention;6) prior art devices are non-isotropic devices, i.e. they require a veryprecise alignment of the article being inspected. These facts imposeconstructive and operative constraints on the apparatus and on theinspection method.

U.S. Pat. No. 4,731,855, to Kyo Suda et al, includes in its Backgroundof the Invention a list of various methods for performing semiconductorwafer inspections, and said list is incorporated herein by reference.One of said methods involves scanning the wafer surface with a laserbeam and analyzing the number and direction of diffraction lights,produced by the pattern edges, by means of a plurality of lightdetection cells arranged cylindrically.

U.S. Pat. No. 4,345,312, to Toshikazu Yasuye et al, discloses a patterninspecting method which comprises picking up an image from an articlehaving a preset pattern whereby to extract the data of the pattern to beinspected, converting said data into a bit matrix of binary values, andcomparing said matrix with a reference matrix representing an idealpattern, to disclose any discrepancy between the pattern of the articleand the ideal one.

U.S. Pat. No. 4,342,515, to Masakuni Akiba et al, discloses aninspection apparatus for determining the presence of foreign matters onthe surface of a wafer, which apparatus includes a beam generatorportion which projects a collimated beam towards the object to inspectit from a side thereof, and a mechanism which senses light reflectedfrom the surface of the object, through a polarizer plate. Such methods,however, are obsolete inasmuch as they cannot be used with today'swafers having a design rule of 0.5 μm or less.

The same principle is used in several prior art methods and apparatus.Thus, in U.S. Pat. No. 4,423,331, to Mitsuyoshi Koizumi et al, the lightreflected from the wafer surface is directed to a photoelectric tube anddefects are detected by the irregularities of the voltage currentoutputted by the tube.

U.S. Pat. No. 4,669,875, to Masataka Shiba et al, makes reference to theaforesaid U.S. Pat. No. 4,342,515, and proposes a method and apparatusbased on the same principle, in which a polarized laser beam irradiatesthe substrate from directions inclined with respect to the perpendicularto its surface and linearly scans said surface; and light reflected fromforeign particles is detected by a polarized light analyzer sand aphotoelectric conversion device.

The aforesaid U.S. Pat. No. 4,731,855 discloses a method of detectingdefects, e.g. foreign particles, in which the diffraction lightreflected from a wafer surface is analyzed by distinguishing betweennormal and abnormal directions. An ideal pattern formed on a waferreflects diffraction lights in determined directions, at certain angles,which are considered normal directions. On the other hand, foreignparticles reflect the light in other, abnormal directions. Reflection oflight in abnormal directions indicates a departure of the pattern formedon the wafer from the real pattern, and therefore possible defects. Inthe invention of this reference, the abnormal direction signals are soapplied as to determine whether they represent a true defect or apractically acceptable defect. Again, this method is obsolete due to thedesign rule of less than 1 μm.

U.S. Pat. No. 4,814,596, to Mitsuyoshi Koizumi et al, applies the saidprinciple of analyzing polarized reflected light to identify defects. Itcites the aforesaid U.S. Pat. No. 4,342,515 as well as Japanese PatentApplications Publication Nos. 54-101390, 55-94145 and 56-30630. In theapparatus of this reference, an S-polarized beam is arranged toilluminate the pattern present on the wafer. Since the irregularities inthe surface of the pattern are sufficiently small, the S-polarized lightis preserved in the reflected light. An analyzer is used to cut theS-polarized light in the path of the reflected light, so that, if thereflected light includes a P-polarized light, this latter is detected bya photoelectric conversion element, indicating the presence of particleson the wafer.

U.S. Pat. No. 4,628,531, to Keiichi Okamoto et al, discloses a patternchecking apparatus, which reveals by a primary selection the presence ofdefects that may be tolerable or not, defined as “candidate defects”.The wafers having such defects are passed to a secondary selection,which distinguishes between those that are not defects in a practicalsense and are acceptable, and those that are not acceptable. Falsealarms, viz. the detection in the primary selection of apparent defects,which are revealed in the secondary selection not to be real defects,are said to be caused, in prior art methods based on the comparison ofpatterns, by an imperfect registration of the patterns to be compared.

Another method of the prior art relates to inspection apparatusemploying a planar array of individually addressable light valves foruse as a spatial filter in an imaged Fourier plane of a diffractionpattern, with valves having a stripe geometry corresponding to positionsof members of the diffraction pattern, blocking light from thosemembers. The remaining valve stripes, i.e. those not blocking light fromdiffraction order members, are open for transmission of light. Lightdirected onto the surface, such as a semiconductor wafer, formselongated curved diffraction orders from repetitive patterns of circuitfeatures. The curved diffraction orders are transformed to linear ordersby a Fourier transform lens. Various patterns of stripes can be recordedand compared. Related discussion can be found in U.S. Pat. No. 4,000,949and 4,516,833.

U.S. Pat. No. 5,699,447 discloses and claims an apparatus whichcomprises first examining means for examining in a first phase thecomplete surface of the wafer with an optical beam of small diameter andfor outputting information, indicating inspected locations on thearticle's surface having a high probability of a defect, storage meansfor storing the output of the first examining means, and secondexamining means for examining in a second phase and with a relativelyhigh spatial resolution only the locations having a high probability ofa defect and for outputting information indicating the presence orabsence of a defect in said locations. The first examination phase iseffected by making a comparison between the pattern of the inspectedwafer and another pattern, serving as a reference pattern; and thesecond examination phase is carried out by a similar comparison toidentify the locations in which the comparison shows such differences asto indicate the presence of a defect.

The methods and apparatus of the prior art have several drawbacks,partly discussed in the cited references, such as errors due to faultyregistration and other causes, false alarms consisting in the detectionof defects that are only apparent, and so on. All of them, further, havethe common drawback of requiring complex apparatus, with high mechanicalprecision, and requiring long operation times and having therefore a lowthroughput.

It is therefore a purpose of this invention to eliminate the drawbacksof the prior art method and apparatus for the inspection of patternedsemiconductor wafers, and particularly for determining the presence ofparticles of foreign substances.

It is another purpose of this invention to provide such a method andapparatus that operate at a much higher speed than prior art apparatusand with a much higher throughput.

It is a further purpose of this invention to detect the defects orsuspected defects of surfaces, particularly of patterned, semiconductorwafers, by a system that does not require comparison of patterns.

It is a still further purpose of this invention to detect said defectsor suspected defects by an inspection or testing of the pixels of thesurface.

It is a still further purpose of this invention to detect said defectsor suspected defects by an analysis of the optical response of thepixels of the surface to a scanning beam.

It is a still further purpose of this invention to provide a wafercontrol method that is not based on a comparison of patterns, but is apixel-based inspection.

It is a still further purpose of this invention to provide a wafercontrol method and apparatus that are completely automatic and eliminatealmost all possibility of human error.

It is a still further purpose of this invention to provide such a methodand apparatus which are highly flexible or, in other words, that can beoperated in such a way as to achieve the precision that is required inany particular processing situation.

It is a still further purpose of this invention to provide a method andapparatus for controlling the semiconductor wafers inline andimmediately recognizing any failures or irregularities in the productionprocess and apparatus.

It is a still further purpose of this invention to provide a method andapparatus that permit to localize on the wafer surface the position ofany suspected defects.

By the expression “vector die-to-die comparison”(abbreviated as VDDC) ismeant, in this specification and claims, an operation the purpose ofwhich is to determine which of the suspected defects represent validpattern data and which represent real defects. The preferred embodimentdescribed herein requires firstly transforming the polar coordinates ofthe wafer inspecting apparatu—hereinafter “the machine coordinatesystem”—to the Cartesian coordinates of a system hereinafterdefined—“the die coordinate system of the wafer”—Then, deriving from thecoordinates that define the suspect pixels' location in the machinecoordinate system the coordinates that define said location in the diecoordinate system. Finally, the VDDC is an operation for discriminating,between suspect data that are actually produced by the wafer pattern andsuspect data that are produced by real contamination by particles—all aswill be fully explained hereinafter.

It is a still further purpose of this invention to provide a method andapparatus for the analysis of surfaces, even if they are not surfaces ofpatterned semiconductor wafers.

It is a still further purpose of this invention to provide an opticalhead which comprises, in a structural unit, all the optical elementsrequired for irradiating the pixels of the surface with the beam usedfor scanning and collecting their optical response in the particularmanner of this invention, as hereinafter described.

It is a still further purpose of this invention to provide an apparatuswhich effects the control of the pixels by a combination of such anoptical head and means for displacing the surface relative to it.

Other purposes and advantages of the invention will appear as thedescription proceeds.

SUMMARY OF THE INVENTION

The invention, both as to method and apparatus, is based on theprinciple of inspecting all or part of the individual pixels of thepatterned wafers under control, without comparing patterns or needingspecific information about the patterns. In other words, the inventionis based on the principle of detecting suspected pixels, viz. pixelsthat show signs of having a defect, particularly the presence of foreignparticles, without reference to the pattern to which the pixel belongsor to the position of the pixel on the wafer and without comparisonbetween patterns. This inventive inspection method is termed herein“design rule check”. Although reference will be made herein to patternedsemiconductor wafers, the analysis of which is the primary purpose ofthe invention, it will be apparent that the invention can be applied ingeneral to the analysis of different surfaces, particularly of anysurfaces not patterned or having patterns the dimensions of which aresimilar to those of wafer patterns, e.g. in the order of microns orfractions of microns.

The method and apparatus of the invention can be used “inline”, viz. issuitable to be integrated with the production process tool, using thesame wafer handling and interface system, and can operate as anintegrated particle monitor to provide a constant check of the wafersproduced, and in this way will detect any irregularities or defects thatmay arise in the production line. Sometimes unforeseen phenomena mayoccur in the production line that are so far-reaching as to render itsfurther operation impossible or useless. It is important to detect suchphenomena, which may be termed “catastrophic”, as soon as possible, andthis invention permits to do so. These inline checks are renderedpossible for the first time in the art by the high speed of thepixel-based inspection method and the moderate cost and footprint of theapparatus.

According to an aspect of the invention, the same comprises a method forthe determination of defects, particularly the presence of foreignparticles, in patterned, semiconductor wafers, which comprisessuccessively scanning the individual pixels, defining the signature ofeach pixel, and determining whether said signature has thecharacteristics of a signature of a faultless or of a defective, orsuspected to be defective, pixel.

In some embodiments of the invention, the determination of thecharacteristics of the pixel signatures is preceded by preliminary stepsof evaluation of the characteristics of the individual signals that makeup the signature, which permit to conclude that certain signaturescannot belong to defective pixels, and therefore require no furtherprocessing, whereby to reduce the amount of data that must be processed.Therefore the method of the invention may comprise defining thesignature of each pixel, evaluating each signal of each signature, and,based on said evaluation, excluding a number of signatures from furtherprocessing. Preferably, the pixels are optically scanned by means of anilluminating beam and their signature is defined by their opticalreaction to the illuminating light. In this case, the variousembodiments of the method of the invention are characterized by thefollowing features:

I—The type of light being used;

II—The physical and geometric parameters of the illumination;

III—The property and/or parameters by which the optical reaction of thepixels, and therefore their signature, is characterized;

IV—The physical and geometric parameters of the detection of saidoptical reaction.

I—The type of light being used

According to the invention, one can use laser beams or light produced byother sources, such as flash lamps, fluorescence lamps, mercury lamps,etc. Laser beams can be produced e.g. by diode lasers and have anywavelength, e.g. 400 to 1300 nm. The choice of the appropriatewavelength can be carried out by skilled persons in any case, so as toproduce optimization for a given material or pattern. Relatively longwavelength (e.g. 600-810 nm) are generally preferred because of the highenergy fluence achievable. Short wavelengths can be preferred fordetecting small particles and for finer design rules. Laser beams canalso be produced by non-diode generators, of any wavelength from IR todeep UV. The illumination radiation may be narrow band or wide band(important for spectral analysis). It can be coherent or non-coherent,polarized or non-polarized. As to fluence, it can be CW, pulsed orquasi-CW. One or a plurality of light beams can be used.

II—The physical and geometric parameters of the illumination

1. The number of the illumination sources can be changed.

2. The geometric placement of the illumination sources can be changed.

3. The size and form of the light source and of the illuminated spot canbe changed.

4. The way in which the illumination light is delivered can be changed.

Important changes can arise from changing the size of the illuminatedspot with respect to a given pattern. A spot of 5 square microns willprovide a completely different set of signatures than a spot of 75square microns, and different discrimination capability. Some usefullight source forms are a point source, a ring source, a large aperturesource, and a line source. It may sometimes be beneficial to illuminatethrough the wafer (or through another article, when such is beinginspected, such as a reticle or some other transparent article) with arelatively large wavelength (more than 1 micron). Thus, one couldilluminate from beneath the wafer and collect the received radiationfrom above. The illumination light can be delivered by optical trains,fiber optics, or other directing elements.

III—The property and/or parameters by which the pixel signatures arecharacterized

1. In this system, the energy of the scattered light is the mainproperty that is being measured.

2. Another property is the height of the surface. This is measured bythe height measurement system.

3. Other properties can also be used successfully for creation of asignature. These are:

3.1. The polarization of the received radiation, in P and S planes. Thisis important, since there are many geometric locations at which thepattern on the wafer induces a well determined polarization, so that acorrectly aligned polarizer would sense only particles.

3.2. The phase of the received radiation.

3.3. The wavelength of the received radiation, which can be tested invarious ways, e.g. by testing for fluorescence or by testing thespectral response of a pixel.

With reference to the polarization of the received radiation, it hasbeen shown (see J. M. Elson, Multilayer coated optics: Guided wavecoupling and scattering by means of interface random roughness, J. Opt.Soc. Am. A12, pp. 729-742 (1995)), that the polarization direction fieldaround a patterned wafer surface, when illuminated with polarized light,exhibits a phenomenon whereby at certain collection angles thepolarization field is well defined. Thus, with a properly alignedpolarizer, the light scattered from the pattern will contribute almostzero energy at said angles. On the other hand, if there is a particle inthe illuminated spot, the light scattered from it is depolarized, andwill contribute significant energy at said angles, whereby the particlewill be clearly detected. It is not necessary to fix precisely thelocation and polarization direction of the detectors that will permit todetect a particle in this way: it suffices to provide a sufficientlyhigh number of detectors, each with a polarizing sheet in front. Theplurality of detectors ensures that some of them will get a responsethat will indicate the presence of a particle. When using polarizedlight, therefore, the method and apparatus of the invention need not bemodified as to the way of delivering the illuminating light anddetecting the light scattered from the wafer pixels, but a polarizershould be placed in front of each detector, no other change beingrequired in the apparatus, and the signal processing algorithms shouldbe modified to take into account the fact that the detectors whichgenerally capture low levels are those placed at the aforesaid angles inwhich the polarization field is well defined, so that if the lightcollected by those detectors has significant energy, the algorithmshould signal the presence of a particle.

IV—The physical and geometric parameters of the detection of the opticalreaction of the pixels

The optical reaction, and therefore the signature of the pixels isdefined by the light scattered by the pixels. The way in which it isdetected can vary widely. It is detected in a plurality of directions,which will be called, for descriptive purposes, “fixed directions”. Eachdirection is defined by a line from the pixel to a point of lightcollection. Therefore, the geometry of the scattered light detection isdefined by the disposition of the points of light collection. Saidpoints may be disposed e.g. in azimuthal symmetry on horizontalconcentric circles (“horizontal” meaning herein parallel to the wafersurface), or in elevational symmetry on vertical or slanted semicircles,or in a flat grid parallel to the wafer surface, or in a semi-sphericalor other vault-like arrangement above the wafer.

In a preferred form of the invention, said signature is defined by anarray of signals, each of which measures the intensity of the lightscattered by the pixel in a direction, and will be called herein“signature component”. The number of directions in which said intensityis measured, and therefore the number of signature components should besufficiently high for the signatures to characterize the correspondingpixels, as hereinafter better explained. The said signals are sampled ata given frequency “f”, which will be called “the sampling frequency”.The period of time between successive samplings, t=1/f, will be called“the sampling period”. The sampling frequency used in carrying out theinvention is preferably very high, in the order of millions of Herz,e.g. 11 Mhz. Each sample generates an array of digital signals, whichdefines the signature of the pixel that was illuminated by the beam atthe moment the sample was taken.

The term “scanning beam” is to be construed herein as meaning a beamthat has a relative motion with respect to the wafer and successivelyimpinges on different points of the wafer. The invention comprisesrelative motions of any kinematic nature and produced by any mechanicalmeans, as long as they cause the spot of the beam to move over the wafersurface. By “spot of the beam” (also called “the beam footprint”) ismeant the area of the wafer illuminated by the beam at any moment, or inother words, the intersection of the beam with the surface of the wafer.In view of the relationship between the wavelength of the scanning beamand the dimensions of the elements of the wafer pattern and of theforeign particles, the light scattered by the wafer is diffracted.

The term “pixel”, as used in this specification and claims, means thearea covered by the spot of the beam at the moment a sampling is carriedout, viz. the moment at which the digital signals, representing theintensity of the light scattered by the wafer in the fixed directions,are determined. Ideally, each pixel should border on the pixels adjacentto it, but this is not necessary for successfully carrying out theinvention. In practice, depending on the character and speed of therelative motion of the scanning beam with respect to the wafer, on thearea of spot size, and on the sampling frequency, adjacent pixels mayoverlap, so that each point of the wafer is examined more than once, or,on the contrary, the adjacent pixels may be spaced from one another, sothat not all the points of the wafer will be examined. One or the otherrelationship between pixels may be chosen, and the relative motion ofthe scanning beam with respect to the wafer may be determined asdesired, taking into account such parameter as the resulting amount ofdata and the speed of the operation.

In order to determine whether a signature has the characteristics of asignature of a faultless or of a defective pixel, any criterion that isadapted to the specific conditions in which invention is carried out,and provides the desired type and degree of selection, can be adopted.The criterion may comprise a comparison between the controlled signatureand a master signature, or the definition of ranges of acceptableparameters in which the parameters of the controlled signature must beincluded, or the position of the controlled signature in a statistics ofsignatures, and so on. A broadly suitable and simple method will bedescribed hereinafter by way of example.

According to another aspect of the invention, at least one source of anirradiating beam, preferably a laser diode, is provided and ispreferably motionless, the controlled wafer is preferably rotated, morepreferably about its center, and is translated (viz. displaced parallelto itself along a straight or curved line), preferably by displacing itscenter in a line that lies in a plane perpendicular to its axis ofrotation, so as to move the spot of the beam over the surface of thewafer, and the light scattered by the wafer is collected in a pluralityof directions. These directions, in which the scattered light iscollected, will be called hereinafter “fixed directions”. The rotarymotion of the wafer has considerable advantages, in particular it iseasy to effect by mechanical means of conventional precision and permitsto achieve very high process velocities and therefore a very highthroughput, while having a small footprint. Although a rotational and atranslational motion of the wafer, the scanning beam being motionless,have been mentioned hereinbefore as preferable, it is the relativemotion of the beam with respect to the wafer that is the determiningfactor, and any manner of obtaining it is equally within the scope ofthe invention. Preferably, in each fixed direction the collected lightis transduced to an electric signal and this latter is converted to adigital signal—a pixel component—by sampling.

In a variant of the above aspect of the invention, a single scanningbeam is provided and the wafer is so moved that said beam scans theentire surface of the wafer. A plurality of lasers, the spot sizes ofwhich substantially overlap, are considered herein as producing a singlescanning beam.

In another variant, the surface of the wafer is partitioned into anumber of zones, a number of scanning beams (preferably equal to saidnumber of zones) is provided, each scanning beam being associated withone of said zones, the inspected wafer is so moved that each beam scansthe wafer zone associated with it, and the light produced by thescattering of each beam by the wafer surface is collected in a pluralityof fixed directions associated with said beam. Typically and preferably,said zones of the wafer, except the central one, which is circular, areannular, concentric rings having similar radial dimensions, and thewafer is rotated and is shifted approximately radially by an amountequal to said radial dimension of the rings. This variant of theinvention shortens the processing times, requires smaller motions of theapparatus elements and permits to define smaller pixels.

In a further preferred form, the process of the invention comprises thefollowing steps:

1—irradiating each wafer with one laser beam or with a plurality oflaser beams;

2—causing a relative motion of each wafer with respect to said beam, ifone laser beam is used, to cause said beam to scan the wafer, and if aplurality of laser beams is provided, to cause each beam to scan a zoneof the wafer associated with it;

3—sensing the light scattered by the wafer in a plurality of fixeddirections, if a single beam is provided, or in a number of suchpluralities associated each with a beam, if more than one beam isprovided;

4—converting said scattered light, in each fixed direction, to anelectric signal;

5—sampling said electric signal at a predetermined sampling frequency,whereby to determine, at each sampling, an array of values, one value ineach fixed direction, associated with a pixel of the wafer;

6—considering each said array of significant values as constituting apixel signature;

7—defining the conditions which must be satisfied by all the pixelsignatures of a faultless wafer;

8—determining whether the pixel signatures of each wafer meet the saidconditions; and

9—classifying the pixels which meet the said conditions, as acceptablepixels and the remaining pixels as “suspect”.

In an embodiment of the invention, a group of beams may be used to scana wafer by focusing them so that all have the same spot on the wafersurface. in this case, the scattered light produced by all the beamswill be collected in the same fixed directions.

Concurrently with the identification of the suspect pixels, theirlocation on the wafer is recorded to permit successive vector die-to-diecomparison. At each moment of the process, the position of the pixelsunder examination is identified in the machine coordinate system. Inthat system, the position of each pixel is defined by the angle by whichthe wafer support has rotated and by the distance of the pixel from thewafer center, or, as may be said, its radial position, which depends onthe displacement which the wafer center has undergone with respect tothe laser beam. Said angular and radial positions constitute the polarcoordinates of the pixels. The position of the pixel on the wafer, onthe other hand, is defined in the die coordinate system, in which apoint is identified by the index of the die it is in and the coordinatesof the point inside the die, with the axes parallel to the principaldirections of the die and the distances measured in microns. The way inwhich the die coordinates of a point are calculated will be describedhereinafter.

In a preferred form of the invention, the signatures of the pixels aretransmitted, together with their coordinates, to a hardware component ofthe apparatus. By “hardware component” is meant herein an electronicdevice having a specific task or a number of specific tasks which can beselected as desired in each case. In general, the hardware component isa specially designed digital electronic device, the task of which is toanalyze the signals and make the preliminary selection between signalsthat represent a valid pattern on the wafer and those that are suspectedto arise from a contaminated spot. The signatures of the suspect pixelsand their coordinates are transmitted further to a software component,which completes the die-to-die comparison. It will be understood that,since the suspect pixels are only a small fraction of all the wafer'spixels, the information thus outputted by the hardware component is asmall fraction of the information received by it.

An embodiment of the invention therefore comprises determining theposition of the apparently defective pixels in the suspect wafers.Another embodiment of the invention comprises measuring the height ofthe pixels. Each type of wafer has a pattern having a given depth. Largeforeign particles often have a height, viz. a dimension perpendicular tothe wafer surface, in excess of said depth of the wafer pattern, andtherefore protrude from said pattern and their presence can be detectedby a height measurement.

The invention further comprises an apparatus for the determination ofdefects, particularly the presence of foreign particles, in patterned,semiconductor wafers, which comprises:

a) a turn table for supporting a wafer and rotating it;

b) a light source and optics for generating at least one light beam anddirecting it onto the wafer;

c) means for shifting the spot of said beam relative to the wafercenter, preferably by shifting the axis of rotation of the wafer;

d) collection optics for collecting the light scattered by the wafer ina number of fixed directions;

e) photoelectric sensors for generating electric analog signalsrepresenting said scattered light;

e) AID converter for sampling said analog signals at a predeterminedfrequency and converting them to successions of digital componentsdefining pixel signatures;

f) means for determining the coordinates of each pixel;

g) a hardware filter for receiving the pixel signatures and theircoordinates and identifying the signatures that are not signatures offaultless pixels, viz. that are signatures of suspect pixels; and

h) a software algorithm for receiving from the filter the signatures ofsuspect pixels, together with the corresponding pixel coordinates, andcarrying out a vector die-to-die comparison.

In a preferred embodiment of the apparatus according to the invention,the light beam is a laser beam. In a more preferred embodiment, themeans for generating a laser beam and the means for collecting the laserlight scattered by the wafer in a number of fixed directions areassociated, in the appropriate geometrical relationship, in a singlestructural unit, herein called “optical head”. An optical head generallycomprises a single laser generator, but if it comprises more than one,the generators are so focused as to produce a single illumination spot.

In said embodiment, therefore, the apparatus comprises:

a) a turn table for supporting a wafer and rotating the same about anaxis of rotation that coincides with the geometric axis of the wafer;

b) means for translationally shifting the axis of rotation of the wafer;

c) at least one optical head;

d) photoelectric means for transducing the optical signals generated insaid optical head to electric analog signals;

e) A/D converter for sampling said electric analog signals at apredetermined frequency and converting them to successions of digitalcomponents defining pixel signatures;

g) a hardware filter for receiving the pixel signatures and theircoordinates and identifying the signatures that are not signatures offaultless pixels, viz. that are signatures of suspect pixels; and

h) a software algorithm for receiving from the filter the signatures ofsuspect pixels, together with the corresponding pixel coordinates, andcarrying out a vector die-to-die comparison.

The optical head is, in itself, an object of the invention.

In an aspect of the invention, the apparatus comprises, in combinationwith mechanical means for supporting and rotating a wafer, optical meansfor substantially isotropically collecting the light scattered by thewafer, and hardware means for taking into account any angulardisplacement of the principal directions of the wafer dies with respectto the wafer support plate. By “substantially isotropically collectingthe scattered light” is meant collecting it at capture angles that areso many and densely distributed that an angular displacement of theoptical collecting means will not significantly change the opticalsignals so collected. In other words, the optical collecting means willbehave approximately as if they were constituted by rings, set in planesperpendicular to the axis of the wafer rotation, uniformly sensing thescattered light at every point thereof. The means for taking intoaccount any angular displacement of the principal directions of thedies, with respect to the wafer support plate, comprises means fortransforming the optical signals actually received to the values theywould have if all the wafers were mounted on their support plate withtheir principal directions set in an invariable, predeterminedorientation.

The signature of any given pixel depends on certain operatingparameters, which must be specified and remain constant in any reductionto practice of the invention. The parameters comprise: a) thecharacteristics of the irradiating light, such as the type of lightsources, the number of such sources, the direction of the irradiatingbeam or beams, their wavelength, their energy fluence, the area of theirspot size, etc.; b) the fixed directions, viz. their number and theirorientation, both as azimuth and as elevation with respect to thesubstrate surface; c) the solid angle within which reflected light issensed by each sensor. Other parameters, referring to the mechanics ofthe invention, will become apparent later. If any of said parameters ischanged, the pixel signatures will change correspondingly. Therefore,said parameters must remain the same in any operation carried outaccording to or for the purpose of this invention. Generally, the largerthe number of fixed directions, the better the resolution of thescattered light and the completeness of the pixel signatures. Structuralconsiderations, on the other hand, prevent using an excessive number ofthem. It has been found that a satisfactory compromise between saidcontrasting factors is to use 16 or 32 fixed directions andcorresponding scattered light collectors. For simplicity ofillustration, in the following description, it will be assumed thatthere are two superimposed rings of fixed directions, each of whichcomprises 16 fixed directions. In each ring, the fixed directions areuniformly spaced in azimuth and have the same elevation angle. The tworings have different elevation angles. By “elevation angle” is meantherein the angle which the direction makes with the plane of the wafer.The plane of a wafer is defined as the plane of its upper surface. Theazimuthal and elevational angles are determined so that all fixeddirections intersect the plane of the wafer at the same point. Theaforesaid fixed direction configuration may also be described by sayingthat said directions lie on two conical surfaces having as their axisthe axis of the wafer and a common vertex, and that they are evenlyspaced on each conical surface.

The scattered light is preferably collected by at least one opticalfiber bundle for each fixed direction, and transmitted to photoelectricdetectors, in each of which a continuous signal is generated. Theterminals of each bundle, which lie on the fixed directions, preferablyabut on one another, so that each ring of optical fiber terminals, lyingon one of said conical surfaces, is continuous. It can be said that theoptical fiber bundles are preferably “interlaced”. The photodetectors,which are conventional apparatus (an example of which is OSD50,manufactured by Centronics), produce continuous electric signals. Thesampling of the continuous electric signal produced by eachphotoelectric detector, can be carried out by apparatus known in the artand available on the market (e.g. AD9059RS, manufactured by AnalogModules) at frequencies of millions of Hz, so that the number of pixelsfor which a signature is obtained is in the order of millions persecond, e.g. 11 Mpix/sec.

The scanning beam generally has an oblong spot size, e.g. having aradial dimension (viz. a dimension parallel or approximately parallel tothe wafer radius) between 5 and 15 microns and a tangential dimension(viz. a dimension perpendicular to the radial one) between 3 and 5microns.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic illustration, in plane view, of the generalfeatures of an apparatus according to an embodiment of the invention;

FIG. 2 is a schematic illustration, in elevational view, of theapparatus of FIG. 1;

FIG. 3 is a schematic illustration, in elevational view, of an apparatusaccording to an embodiment of the invention;

FIG. 4 is a schematic plan view of another embodiment of the invention;

FIG. 5 is a schematic plan view of a variant of the embodiment of FIG.4;

FIGS. 6a and 6 b are schematic vertical cross-sections of twoembodiments of optical head;

FIG. 7 is a plan view, from the bottom, of said optical head, at agreater scale;

FIG. 8 is a block diagram generally illustrating the phases of thescattered light processing according to an embodiment of the invention;

FIG. 9 is a block diagram generally illustrating the analog processingunit of an apparatus according to an embodiment of the invention;

FIGS. 10, 11 and 12 an embodiment of the signal processing;

FIG. 13 illustrates the die coordinate system;

FIGS. 14(a), (b) and (c) and FIG. 15 schematically illustratealternative dispositions of the scattered radiation collectors;

FIGS. 16 and 17(a), (b) and (c) illustrate a method and apparatus forheight measurement; and

FIG. 18 is a conceptual flow chart exemplifying the vector die-to-diecomparison according to the preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 schematically represent an apparatus according to anembodiment of the invention. Numeral 10 indicates a wafer that is beinginspected. The apparatus used for the inspection comprises a stagehaving a wafer support. The wafer is placed on said support, which inthis embodiment is a support plate 11, which is rotated about shaft 12by mechanical means, not shown as being conventional. A laser source isshown at 13 in its central position, above the axis of shaft 12.However, more than one source could be provided and any source could beplaced at an angle to the axis of shaft 12, to provide the requiredillumination of the wafer, depending on the type of wafer underinspection. In FIG. 2 one such additional laser source is shown, by wayof illustration, oriented at an angle β from the plane of the wafer.Mechanical means, not shown as being conventional, translate the shaft12, viz. shift it, while maintaining it parallel to itself, so that anypoint thereof moves in a straight or curved line that lies in a planeperpendicular to the axis of the shaft. Consequently the wafer 10 isalso translated parallel to itself so that its center of the wafer isshifted in a plane perpendicular to the axis of the shaft. By“translatory motion” is meant any motion of a body in which the bodydoes not rotate, but is displaced without rotation along any straight orcurved line. The translational displacement of shaft 12 and wafer 10 isrectilinear, but it need not be: if desired, it could follow a curvedpath.

Additionally, the laser source or sources may not be stationary and thecorresponding spot or spots on the wafer surface may move in a waysimilar to the motion of a needle on a phonograph disk, viz. swing alongan arc of circle passing through the center of the wafer. Consequently,since the laser source 13 remains stationary, the spot of the scanningbeam is displaced on the wafer from the periphery of the wafer to itscenter and/or vice versa, and, possibly but not necessarily, along orapproximately along a radius of the wafer 10. Preferably, the wafer isrotated at a V_(r) of 5000 rpm and displaced radially at a speed L of0.01-0.5 cm/sec.

The light of beam 13 is scattered in the fixed directions. The fixeddirections may be azimuthally or elevationally distinct. In theembodiment illustrated, they are azimuthally distinct and arranged in 2rings of 16 each. In each ring, the said directions are symmetricallyarranged about the wafer and slanted at an elevation angle α₁ for thelower ring and α₂ for the upper ring, from the plane of the wafer. Theangle α₁ is selected from 8 to 15 degrees, and the angle α₂ from 15 to30 degrees. This arrangement, however, is merely an example, and can bevaried as desired, as will be better explained later.

In each fixed direction, the scattered light is collected by an opticalfiber terminal—15 in the lower ring and 16 in the upper ring.Preferably, the generator of the beam 13 and the optical fiber terminalsare structurally associated in an optical head. Each optical fibertransfers the collected light to a photodetector—17 in the lower ringand 18 in the upper ring.

Each photodetector outputs an electric signal, which is transmitted toan electronic circuit, not shown in the drawing, which samples theelectric signals and outputs, for each fixed direction, a digital signalcorresponding to the intensity of the light collected by thecorresponding optical fiber. The sampling frequency f may be, e.g., 11MHz. The spot size of the scanning laser beam, in this example, has anapproximately elliptical shape, with a longer diameter of 15 μm and ashorter diameter of 5 μm.

FIG. 3 is a further schematic illustration, in elevational view, of amachine according to an embodiment of the invention. The machinecomprises a frame 20, on which a mechanical assembly, generallyindicated at 21, is supported. The mechanical assembly comprises a motorassembly 23, which rotates a plate 22 that supports the wafer. Numeral24 schematically indicates means for translationally displacing saidmotor and plate. Numeral 25 generally indicates a scanning system, whichactuates a scanning head 26, containing the laser sources, the opticalfibers for collecting the scattered light, and the photoelectricdetectors. Block 27 schematically indicates the electronic components ofthe machine, which receive the output of the photodetectors throughconnections (not shown) and process it as herein described.

It should be noted that, besides the aforementioned rotary andtranslational motions of the wafer supporting plate, which occur duringthe scanning of the wafers, different motions are required for carryingout the stages of loading and unloading the wafers. Additionally, anautofocusing mechanism is preferably provided for focusing theilluminating beam, and such mechanisms (too) are well known to personsskilled in the art and need not be described

FIG. 4 is a plan view schematically illustrating another embodiment ofthe invention, which comprises a plurality of optical heads. The wafer30 is ideally divided into a number of zones constituted by concentricrings and a central circle. Only three ring zones—31, 32 and 33—inaddition to central circular zone 34, are shown in the drawings forsimplicity of illustration, but in practice there may be more. Anoptical head, comprising a scanning beam and an array of optical fibersensors in the fixed directions, is provided for each zone. In eachfixed direction the scattered light is collected by an optical fibersensor. In this embodiment, the scanning beam and the optical fibersensor of each zone are mounted on a common support, to define anoptical head, hereinafter described. The four optical heads, which areidentical, are indicated by numerals 35, 36, 37 and 38. In FIG. 4 theoptical heads are shown as being one for each zone and successivelyaligned along a radius of the wafer, but this is merely a schematicillustration. The heads may not be aligned along a radius, but, forexample, may be staggered and partially overlap, so that some circlesdrawn on the wafer surface may cross more than one head. Such anarrangement is schematically indicated in FIG. 5, wherein a plurality ofheads, schematically indicated at 39, are staggered generally along aradius of a wafer 30, to cover an equal number of zones not indicated inthe figure. Said arrangement particularly applies to optical heads thatcomprises CCD detectors, that will be described hereinafter withreference to FIG. 15.

The purpose of these embodiments is to cause a plurality of pixels to beilluminated and checked concurrently, whereby the process is acceleratedand the throughput increased; and further, to limit the translationaldisplacement of the wafer to a fraction of what it would be if a singleoptical head were to scan the whole wafer, simplifying the mechanics ofthe machine and reducing its footprint. The translational displacement,as in other embodiments of the invention, need not be radial, but mayfollow a differently directed straight path or a non-straight path, asmay be more convenient in view of the mechanics of the apparatus. Anydisposition of optical heads and/or wafer translation that will servethe purpose of a complete scanning by convenient mechanical and opticalmeans can be adopted.

FIG. 6a is a vertical cross-section of one of the optical heads, ofwhich FIG. 7 is a plan view, from the bottom, at a larger scale. It isassumed to represent one of the optical heads 35 (or 39) but couldrepresent any other optical head. It could also represent the opticalhead of an apparatus which includes only one such head, the wafer beingdisplaced in such a way that the single head scans its entire surface.Head 35 comprises a base 40, which has at its bottom, a central recessor cavity 41 that is arc-shaped, e.g. approximately semi-spherical, andhas a bottom, viz. its opening, that will be parallel to the plane ofwafer when the head is used. Base 40 is mounted in a case 45, supportedin the machine in any convenient way, not illustrated. In base 40 aremounted the laser source 42 and two circular arrays of optical fibers 43and 44, disposed one above the other at different angles so as toconverge onto the center of recess 41, where the pixel that is beingexamined is located. It is seen in FIG. 7 that the terminals of saidoptical fibers, one of which is indicated by numeral 47, whichconstitute the intersection of said fibers with the surface of cavity41, are adjacent to one another, so that each array of said fibers formsa continuous circle at the surface of cavity 41. While two fibers 43 andtwo fibers 44 are shown in the cross-section of FIG. 6a, each arraypreferably comprises, in this embodiment, 16 optical fibers, which aregathered, in this embodiment, into two bundles 46 for connection tophotodetectors, not shown. Further, a plurality of laser sources, e. g.placed at different angles to the wafer plane, and more than twocircular arrays of optical fibers could be comprised in an optical head.No matter how many rings of fiber terminals are provided, said terminalsare preferably disposed in each ring at uniform angular distances fromone another and are so slanted that all of their axes pass through acommon point, which is the center of the bottom of cavity 41 and will bethe center of the portion of the wafer surface exposed by said cavity,when the optical head is superimposed to a wafer surface to carry outthe process of the invention.

FIG. 6b schematically illustrates in vertical cross-section, at a largerscale, the optical components of another embodiment of optical head.Said head comprises three laser sources 42 a, 42 b and 42 c, the firstoriented perpendicularly to the bottom of cavity 41 and the others atdifferent slants thereto, to provide an improved illumination. Twocircular arrays of optical fibers 43 and 44 are provided in this head aswell, and their terminals 48 and 49 form two circles about cavity 41. Itshould be appreciated that the laser source 42 can be situated remotelyfrom the optical head 35, and that the laser beam can be transmitted tothe head using, for example, other optics similar to collection opticfibers 43 and 44.

FIG. 8 is a block diagram generally illustrating the phases of theprocess by which the scattered light from a wafer surface is processedaccording to an embodiment of the invention. The process is illustratedwith respect to an optical head, as hereinbefore illustrated. If theapparatus of the invention comprises a plurality of optical heads, thesame operations are carried out with respect to each of them. If theoptical components of the apparatus are not combined in an optical head,and no matter how they are combined, what will be said about an opticalhead will apply to the processing of the light scattered by the wafersurface carried out in the stages shown in FIG. 8. It is assumed forillustrative purposes only that the optical head unit 50 includes anillumination apparatus, two rings with 16 optical fibers each, whichdetect the light scattered in as many directions (which constitute thefixed directions), and any supporting subsystems that it is desired tointroduce, such as auto-focus mechanism, lenses, etc. The opticalcomponents operate as hereinbefore described.

The analog processor unit 51 is responsible for detecting the opticalsignals from the optical fibers, transducing the signals into electricpulses, amplifying said electric signals, applying a correctioncomputation to make sure that all the detectors (32 in this example) areproperly calibrated with respect to the inspected surface and withrespect to each other, and finally converting each electric analogsignal into a digital signal, preferably with 8 bits.

That is, each sample or pixel signature component is a digital signalconstituted by a word having a sufficient number of bits to provideadequate information. E.g., the word may be composed of 8 bits,providing 256 levels of scattered light intensity. Each ring of thephotodetectors will then contribute 16×8=128 bits with a frequency “f”,which, in this embodiment, is assumed to be 11 Mhz.

The digital signals are transferred as output to a signal processor unit52. The resulting pixel signature outputs must be evaluated according toa predetermined criterion. While, within the scope of this invention,many criteria and corresponding algorithms could be used, according tocases and to the choice of the expert person, a simple criterion willlater be described by way of illustration.

The signal processor unit 52 is responsible for the first stage of thedata reduction. It receives the input signals (32 in this example) fromthe analog processor unit, at the clock rate of the system (e. g. 11MHz). It outputs only a small percentage of these data to the nextstage. The signal processor unit is a custom designed electronic subunitthat can handle very wide input data at a high rate. It is also capableof employing several different reduction algorithms, switching betweenthem as the application requires. The pixels, the data of which aretransmitted out of the signal processor, are henceforth termed “suspectpixels” or “suspects”. The signal processor is designed to transmit asmall fraction of the inspected pixels to the following unit. This datais communicated via a FIFO bank (to coordinate the communication rates),through a standard bus, such as a PCI bus, to the main CPU of thesystem.

The defect detection unit 53 is a software module running on the mainCPU of the system. It receives suspect pixel data from the signalprocessor unit. Its responsibility is to separate the valid pixels fromthe defective ones and output the defect list as the final product ofthe system.

Considering now the components of the block diagram of FIG. 8individually, the preferred structure of the Optical Head Unit 50 hadbeen discussed hereinbefore.

The internal structure of the Analog Processor Unit 51 of this exampleis schematically illustrated in FIG. 9. The unit is composed of a numberof identical channels, one for each optical fiber, three of which aresymbolically indicated in perspective relationship. Each channel (out ofthe 32 of this example) comprises a detector 55 that transduces thelight signal into electric current. The electric signal is thenamplified in a preset amount in a preamplifier 56. The signal is thenfurther amplified with an amplifier 57 that has variable gain andoffset. This allows the system to adapt to varying substrates,illumination angles and parameters such as wavelength, intensity, etc.,and also allows calibrating the several channel signals to respondequally. This insures the isotropy of the whole optical channel, so thatif the same pixel is observed from varying angles (for example, afterrotation of the wafer), the same signature will be obtained from all thesignals. It is also a prerequisite for all subsequent treatment of thesignals. The last block of this Analog Processor Unit is an A/Dconverter 58 that performs analog to digital conversion preferably at 8bits. Thus, in this example, the output of Analog Processor Unit 51 is32 signals, each with 8 bits, at a system clock of 11 MHz.

The Signal Processor Unit 52 is a specially designed digital electronicdevice, the task of which is to analyze the signals and make thepreliminary selection between signals that represent a valid pattern onthe wafer, and those that are suspected to arise from a contaminatedspot. Such suspected signals are passed on to the next analysis step.Preferably, this unit should be reconfigurable to apply variousalgorithms for the discrimination between valid and suspect pixels,changing the algorithm according to the demands of the application. Theoperation of this unit will be explained in algorithmic building blocks.The implementation of the algorithms as a hardware device is well knownto electronic engineers skilled in the art of designing modern digitalsignal processing boards, especially of the kind that is based on FPGA(Field Programmable Gate Array) and DSP (Digital Signal Processor)technology. The details of this implementation are not part of theinvention and need not be discussed herein.

While a variety of algorithms may be devised and used by skilledpersons, a specific algorithm will be described by way of example, withreference to FIG. 10.

The digital signals from Analog Processor Unit 51 (32 in this example)firstly enter into a bump detector 61, each signal independently of theothers. The bump 62 is graphically illustrated in FIG. 11. It isdetected with the following operator, that requires three parameters:total width 63, central width 64 and a ratio threshold 65 between thebrightness peak 66 of the pixel currently under control and the highestbrightness peak 67 in the filter's domain that is outside a centralpatch. By “width” is meant the number of data taken at one time. Thevalues of the two peaks are compared in threshold comparator 69 todetermine their ratio. If the determined brightness ratio exceeds somethreshold, predetermined on the basis of experience, the pixel isretained in the signal, otherwise, it is zeroed. This is done for eachof the 32 signals, and the result is again a set of 32 signals. The mostcommon parameters for this operator have been found to be: filterwidth=11 pixels, central width=5 pixels and ratio threshold=1.3.

If at least one of the 32 signals produced by a pixel has a high bump,viz. its brightness ratio is above the threshold, all 32 signalsrelative to said pixel are passed on to this estimator for thestatistical evaluation. Therefore, in this embodiment, and optionally ingeneral, bump detection is a first preliminary stage filter performedprior to the vector die-to-die comparison. However, the number ofsignals relative to each pixel that have a brightness ratio above thethreshold (the output of bump detection) gives an indication as towhether the pixel is likely to be considered suspect. For instance, asingle signal having a high bump may be due to the wafer pattern,whereas a high number of such signals is probably due to a particle. Thedecay rate estimator 68 (see FIG. 10) analyzes the 32 signals relativeto each pixel (the output of bump detection) and provides somestatistical indicator of their values. Therefore, in this embodiment,and optionally in general, decay rate estimation is a second preliminarystage of the vector die-to-die comparison, successive to bump detection.The algorithm uses three parameters: central value percentile p, widthfactor w, and threshold s. The computation is as follows: Sort the 32values. Pick the p percentile value. Sum all values that are between p*wand p/w. The sum is sent to a threshold comparator. The result of thecomparator is 1 or 0, according to whether the sum is greater or smallerthan s. Common values have been found to be p=w=0.5. The value of s isvariable, and has to be empirically determined. This procedure isillustrated by an example in FIG. 12, wherein numeral 70 indicates acurve which interpolates all the percentile values. The area 71 (markedin black) under curve 70, between twice the center value and one halfthe center value, is compared to an empirical threshold.

Whereas the input to the signal processor unit is synchronous data atthe system's clock, the frequency of the output is on the average 2-4orders of magnitude smaller. A standard interface with some memory isdesigned into the system to handle potential peaks of activity, and topush the data down a PCI bus to the host computer (e. g. Pentium II byIntel Corporation Limited). Each output suspect feature is built ofcoordinate data and type data. The coordinate data is provided by themechano-electronic subsystem in polar coordinates (ρ and θ) and can betranslated, using the registration transformation described herein, towafer coordinates. The type data is in the case of the algorithmdetailed above the output of the decay rate estimator (the number thatwas sent to the threshold comparator). This is an indication of thestrength of the detection, or, in other words, of the detectioncertainty.

The Defect Detection Unit 53 is a software module whose job is toreceive the data relative to all the suspect pixels from the previousstage, and find out which of them represent real defects, viz. to carryout the vector die-to-die comparison (VDDC), which follow thepreliminary stages of bump detection and decay rate estimation, if thesehave been carried out. The VDDC operation comprises the following:

1. Transforming the polar coordinates of the machine to the Cartesiancoordinates of the die coordinate system.

2. Deriving from the coordinates that define the suspect pixels'locationin the machine coordinate system the coordinates that define saidlocation in the die coordinate system.

3. Marking in the die coordinate system the position of all suspectpixels.

4. Discriminating between suspect pixels that are due to the waferpattern, and therefore do not represent a defect, and suspect pixelsthat are not due to it and therefore do represent a defect, inparticular those deriving from foreign particles.

FIG. 13 illustrates how the coordinates that define the suspectpixels'location in the die coordinate system are derived from theircoordinates in the machine coordinate system. When a new wafer 100 isplaced, generally by means of a robot, on the rotary plate of theinspection apparatus of the invention, the orientation of the wafer isunknown and it may be miscentered by up to 1 mm. It is important for thelater parts of the inspection procedure, and of course for the output ofthe list of defective pixels, to be able to describe the location ofeach pixel on the wafer with the wafer's natural frame of coordinates.The wafer is composed of many identical dies 101, each one of which isdestined to constitute (if not faulty) a semiconductor device or part ofsuch a device, such as a CPU or a memory chip. The dies are separated bythe scribe lines, along which the wafer will be cut when ready, whichform two perpendicular families of lines, that can be called, forconvenience of illustration, avenues 102 and streets 103. Avenues andstreets, collectively, can be called “the principal directions of thedie”. The avenues are considered as oriented from “south” to “north” andthe streets from “west” to “east”. At one point on the circumference ofthe wafer there is a small recess, called a notch 104, which defines thewafer's “south”.

The procedure of defining the wafer map and registering it withreference to the machine coordinate system, is carried out as follows:

1. A short pre-scan is carried out, typically by rotating the machineplate by 100 turns, spanning about 20 mm width of the externalcircumference of the wafer.

2. The notch is detected by its known, typical signature.

3. Streets and avenue pixels are detected by their specific signatures,which are determined by their being mirror-like in most of their area.

4. The detected notch and street and avenue pixels are transmitted tothe CPU that controls the procedure.

5. A registration algorithm receives the said input and computes theangle of rotation of the wafer's coordinate system with respect to themachine's coordinate system, and also the locations of the streets andavenues.

This allows a map of the wafer to be constructed (when a new wafer isintroduced) or a registration transformation to be computed (if the mapis already known). If the wafer is a bare wafer, then only the notch canbe detected and a registration transformation of lower accuracy can becomputed, using the location of the notch. Such registrationtransformation can be carried out by means of known algorithms, e.g. bythe randomized Hough transform technique—see L. Xu, E. Oja and P.Kultanen, A new curve detection method, Randomized Hough Transform(RHT), Pattern Recognition Letters, vol. 11. no. 5, May 1990, pp.331-338, Elsevier Science Publishers B.V. (North-Holland—or by otheralgorithms easily devised by expert persons.

The inventive vector die-to-die comparison (VDDC) will now be described.Conceptually, unlike the conventional die-to-die comparison, whereineach (x_(i),y_(i)) location on a die is compared to the correspondinglocation on the preceding and following die, the VDDC “stacks” all thedies and checks to see whether a suspect (x₀, y₀) location appears in acorresponding location in more than one die. The entire operation,including the coordinates transformation, can be better understood withreference to FIG. 18. FIG. 18 depicts a defect map in the form of awafer, 80, having a plurality of dies 82 thereon, and having “suspected”defect locations, each marked with an “x”.

In the apparatus according to the preferred embodiment, the suspectedlocations are provided in polar coordinates, i.e., (r_(j), θ_(i)) pairs.Therefore, the suspected locations'coordinates are first transformed tothe cartesian coordinates of the wafer, i.e. (x_(j), y_(i)) pairs, andthence to the Cartesian coordinates of the corresponding dies, i.e.(x_(kj), y_(kj)) pairs (k standing for the die and j standing for thecoordinate within the die). Once the (x_(kj), y_(kj)) pairs of all thesuspected locations within the wafer have been obtained, the dies are“stacked” to see whether any suspect location (x_(kj), y_(kj)) appearsin more than one die. Here, much information about the suspect locationscan be obtained. For instance, if a particular (x_(kj), y_(kj)) pairappears in all the dies, it is likely to be a feature of the diestructure and not a defect.

Also, as is known in the art, the patterns on the wafers are created bya process called photolithography, which uses reticles having thedesired pattern thereupon. It is known to use reticles which have, forexample, a multiple of four die patterns thereupon. Thus, if during theVDDC it is determined that a particular (x_(kj), y_(kj)) pair appears inevery fourth die, it is likely that the defect has been transferred froma defective reticle. Thus, this information can be used to setthresholds and other filtering mechanisms for the VDDC.

As can be seen from the above, using the VDDC discrimination is noweffected between two types of suspect data:

a) Suspect data that are actually produced by the wafer pattern, butappear as if they were due to the presence of particles (and thereforeare detected as indicating such presence). These will appear in many orall of the dies. Thus, in die coordinates, one will see numerousappearances of suspects at the same location. All the suspects thatappear at that location are discarded. It may be advantageous to filterthe data in some way before the VDDC, for example by utilizing a methodthat recognizes that a group of points form together some specificgeometric configuration, for example line segments. This group can thenbe considered as a legitimate pattern of the wafer and filtered out ofthe set of suspect points. This allows the VDDC to operate on a smallerset of isolated points and thereby to achieve better performance.

b) Suspect data that are produced by real contamination by particles.These appear essentially only once on the die map.

The detailed construction of software that implements this algorithm isa routine task for a skilled algorithm designer.

It should be appreciated, of course, that rather than using theinventive VDDC, one may choose to perform a conventional die-to-die orcell-to-cell comparison. Even if such an approach is taken, theinventive system reduces processing time, since, unlike conventionaldie-to-die systems wherein all the pixels are compared to their nearneighbors, only the suspect pixels flagged by the Signal Processor Unit52 need to be compared to their near neighbors. Of course, in such acase, each time the Signal Processor Unit 52 flags a suspect pixel, thenear neighbor pixels need to be stored in the memory for the die-to-diecomparison.

FIGS. 14 (a), (b) and (c) schematically illustrate the disposition ofscattered radiation detectors above the wafer surface and notperipherally, as in the previously described embodiments. The fixeddirections, therefore, are elevationally and not azimuthally spaced. Thewafer is indicated at 70. In FIG. 14(a) only one semicircular detectorring 71 is shown, along which detectors 72 are disposed. Such a ringwill detect radiation scattered on a plane perpendicular to the waferplane, at different elevational angles on said plane. In FIG. 14(b)several semicircular rings 73 of detectors are provided, in any desirednumber, though only three are shown for convenience of illustration inthe drawing. The rings are on different planes, differently slanted withrespect to the wafer plane. If there is an odd number or rings, thecentral one will be on a plane perpendicular to the wafer plane. FIG. 14(c) shows two rings 74 of detectors, disposed on two planesperpendicular to the wafer plane and perpendicular to each other. Inthis case too, any desired number of detector rings could be provided.It is clear that the geometric arrangement of the detectors can bechanged as desired.

In a further embodiment of the invention, schematically illustrated inFIGS. 5 and 15, pixel signatures are determined by components that aredetermined by measuring elevation angles scattering rather thanazimuthal angles scattering signature. In this case, it is possible andpreferable to use line CCD detectors and line laser diode bars placed asa fan. Thus FIG. 15 shows a wafer 110 with a set of linear CCD elements111. For instance, there can be used CCDs with 1000 detectors each,arranged as a fan with 10 units. One of these elements can be replacedwith a laser diode bar. This provides the required illumination,together with the versatility needed to set the illumination angle tosuit each particular case. With 1000 detectors, each capturing theenergy from a pixel with radial dimension of 15 microns, one would cover15 mm. of the wafer's radius, that is, about 10%. Therefore an apparatusaccording to the invention with e.g. 10 static heads could be used inplace of an apparatus with one head moving radially across the wafer.

The depth measurement is optional and, when effected, can be carried outby devices known in the art as to themselves, although never beforeapplied to the testing of semiconductor wafers. Such devices are used,e.g. as autofocusing mechanisms in the Compact Disk art. See for exampleH. D. Wolpert, “Autoranging/Autofocus—A Survey of Systems”,Photonics—Spectra, June p.65, Aug. p.127, Sep. pp. 133-42, Vol. 21, Nos.8 and 9 (1987). Schematically, they may be constituted and operate asillustrated by way of example in FIGS. 16 and 17. In FIG. 16, 50designated a portion of a patterned wafer surface, on which a largeparticle may have been deposited. A laser diode 52 emits a beam whichenters a diffraction grating 53, which converts the beam into a centralpeak plus side peaks. The resulting three beams go though a polarizingbeam splitter 54, which only transmits polarization parallel to a plane,which in this example is assumed to be the plane of the drawing. Theemerging, polarized light is collimated by collimator 55. The collimatedlight goes through a ¼ wave plate 56. This converts it into circularlypolarized light. The circularly polarized light is then focused onto thewafer through objective lens 57. The light reflected by the wafer goesback into the objective lens 57 and then passes once again through the ¼plate. Since it is going in the reverse direction, it is polarizedperpendicularly to the original beam, viz, perpendicularly, in thisexample, to the plane of the drawing. When the light hits the beamsplitter 54 once again, it is reflected through a focusing lens 58 and acylindrical lens 59 and is imaged on a photodetector array 60. If theobjective lens is closer to the reflecting area of the wafer than thefocal plane 61 of the objective lens 57, an elliptical image is createdon the photodetector array 60, as shown in FIG. 17(a). If it is fartheraway than said focal plane, another elliptical image is created,perpendicular to the first one, as shown in FIG. 17(b). If thereflecting area of the wafer is at the focal length of the objectivelens, the cylindrical lens does not affect then image, which iscircular, as shown in FIG. 17(c). Therefore, if the pattern of a waferproduces a circular image, due to the lands of the pattern, viz, theplane of the wafer, being at the focal plane of the objective lens, andwhen a given pixel is illuminated an elliptical image is formed, thiswill indicate the presence of a particle the size of which cause it toproject above the pattern lands. The displacement signal of theobjective lens, required to reestablish a circular image, can give ameasure of the amount by which the particle projects above the plane ofthe wafer.

It should be noted that, while the invention has been described andillustrated on the assumption that the surface to be analyzed is theupper surface of a body, in particular of a wafer, and therefore theirradiating beam is directed downwardly onto it, the supporting shaft islocated below it, and in general all parts of the apparatus conform tothis geometric orientation, the apparatus could be differently oriented,and, e.g., overturned, so that that the surface to be analyzed be thelower surface of a body, in particular of a wafer, with all theattending structural consequences.

While examples of the method and apparatus of the invention have beendescribed by way of illustration, it will be apparent that the inventionmay be carried out with many variations, modifications and adaptationsby persons skilled in the art, without departing from its spirit orexceeding the scope of the claims.

What is claimed is:
 1. A method for the analysis of patternedsemiconductor wafers, which comprises the steps of: (a) irradiating eachwafer with a laser beam; (b) causing a relative motion of each waferwith respect to said beam, to cause said beam to scan the wafer; (c)sensing, in an array of fixed directions, the light scattered by thewafer; (d) for each fixed direction, converting said scattered light ofstep (c) into an electric signal; (e) sampling said electric signal at apredetermined sampling frequency to determine an array of values witheach sampling, wherein each value of the array corresponds to one of thefixed directions, such that the sampling defines a pixel of the wafer;(f) defining a pixel signature from said array of values of step (e);(g) defining conditions for a predetermined number of pixel signatures,such that the pixel signature conditions must be satisfied to considerthe wafer faultless; (h) determining whether the pixel signatures ofeach wafer meet the conditions of step (g); and (i) perfomingpixel-based inspections, without requiring reference pattern data, toclassify the pixels which meet the conditions of step (g) as acceptablepixels and to classify the remaining pixels as “suspect” pixels.
 2. Themethod according to claim 1, wherein after defining the signature ofeach pixel in step (f), at least one signal of each pixel signature isevaluated to determine whether to exclude further processing for anumber of pixel signatures.
 3. The method according to claim 1, furthercomprising the steps of: (j) conducting a die-to-die surface inspectionof a plurality of dies in said semiconductor wafer; (k) obtaining diecoordinates of suspect pixels in each of said plurality of dies; and (l)for each of said suspect pixels, determining whether a suspect locationappears in a corresponding location in more than one die.
 4. The methodaccording to claim 1, further comprising the steps of: conducting a bumpdetection to identify suspect pixels; obtaining die coordinates ofsuspect pixels in each of said plurality of dies; and for each of saidsuspect pixels, determining whether a suspect location appears in acorresponding location in more than one die.
 5. A method for theanalysis of patterned semiconductor wafers, which comprises the stepsof: (a) dividing a surface of each wafer into a number of zones; (b)irradiating each wafer with a number of laser beams, each associatedwith one of said zones; (c) causing a relative motion of each wafer withrespect to said beams to cause said beams to scan the zones of thewafer; (d) sensing, in a number of arrays of fixed directions, the lightscattered by the wafer, wherein each array is associated with a beam;(e) for each fixed direction of each array, converting said scatteredlight of step (d) into an electrical signal; (f) sampling said electricsignal at a predetermined sampling frequency to determine a number ofarrays of values with each sampling, wherein each number of values ineach number of fixed directions defines a pixel of the wafer; (g)defining a pixel signature from each said array of values of step (f);(h) defining for a predetermined number of pixel signatures, such thatthe pixel signature conditions must be satisfied to consider the waferfaultless; (i) determining whether the pixel signatures of each wafermeet the conditions of step (h); and (j) performing pixel-basedinspections, without requiring reference data patterns, to classify thepixels which meet the conditions of step (i) as acceptable pixels and toclassify the remaining pixels as suspect pixels.
 6. The method accordingto claim 5, further comprising the step of subjecting each wafer whichcomprises suspect pixels to vector die-to-die comparison.
 7. The methodaccording to claim 5, further comprising the step of measuring an amountby which the pixels project above a plane of the wafer to detect largeforeign particles.
 8. The method according to claim 6, wherein thevector die-to-die comparison comprises the steps of: transformingsuspect pixel locations on the wafer defined by polar coordinates tosuspect pixel locations defined by Cartesian coordinates of a diecoordinate system; marking positions of all suspect pixel locations inthe die coordinate system; and discriminating between suspect pixelswhich are caused by the wafer pattern and do not represent a defect andsuspect pixels which are not caused by the wafer pattern and represent adefect.
 9. Apparatus for the determination of defects, particularly thepresence of foreign particles in patterned semiconductor wafers, whichcomprises: a stage having a support; a laser source and opticsgenerating a laser beam and directing it onto the wafer; collectingoptics for collecting the laser light scattered in a number of fixeddirections by the wafer; photoelectric sensors for generating electricanalog signals representing said scattered light; an A/D converter forsampling said analog signals at a predetermined frequency and convertingsaid analog signals to successions of digital components defining pixelsignatures; a first selection system which receives the pixel signaturesand their coordinates and performs pixel-based inspections, withoutrequiring reference pattern data to identify the pixel signatures ofsuspect pixels; and a second selection system which receives the pixelsignatures of suspect pixels from said first selection system, togetherwith the corresponding pixel coordinates, and verifies whether eachsuspect pixel is indeed a defect.
 10. The apparatus according to claim9, wherein the stage comprises a turn table such that scanning isaccomplished by shifting the axis of rotation of the turn table.
 11. Theapparatus according to claim 9, wherein said second selection systemfurther comprises a comparator which verifies whether each suspect pixelis a defect by comparing a plurality of dies to determine whether asuspect location appears in a corresponding location in more than onedie.
 12. An apparatus for the determination of defects in patternedsemiconductor wafers, which comprises: a stage having a wafer support; alaser source generating a laser beam; at least one optical head designedto transmit the laser beam onto the wafer, said at least one opticalhead having a plurality of collection fiber optics arranged therein;photoelectric sensors for transducing light collected by the collectingfiber optics into electric analog signals; an A/D converter for samplingsaid electric analog signals at a predetermined frequency and convertingsaid analog signals to successions of digital components that definepixel signatures; selection hardware adapted to receive the pixelsignatures and their coordinates and performs pixel-based inspections,without requiring reference pattern data to identify the pixelsignatures of suspect pixels; and a microprocessor responsive toselection software to receive from said selection hardware the pixelsignatures of suspect pixels, together with the corresponding pixelcoordinates, and evaluating said suspect pixels to single out falsealarms.
 13. Apparatus according to claim 12, wherein the optical headcomprises at least one laser generator and wherein said collection fiberoptics comprises two superimposed rings of optical fibers for collectingthe light scattered by the wafer.
 14. Apparatus according to claim 13,wherein each ring comprises 16 optical fibers, uniformly spaced inazimuth and having the same elevation angle, the two rings havingdifferent elevation angles.
 15. Apparatus according to claim 13, whereinthe laser generator produces an oblong spot size.
 16. Apparatusaccording to claim 12, wherein the selection hardware is a customdesigned electronic circuit comprising a bump detector, a decay rateestimator and a threshold comparator.
 17. The apparatus according toclaim 12, further comprising a depth measuring device adapted to measurean amount by which each pixel projects above a plane of the wafer. 18.The apparatus according to claim 12, wherein said at least one opticalhead comprises: a cavity exposing the surface under control; at leastone laser source; and a plurality of optical fibers having terminalssymmetrically disposed about said cavity.
 19. The apparatus according toclaim 18, wherein the optical fiber terminals of said optical head aredisposed in a plurality of superimposed rings that are evenly spaced ineach ring and have axes passing through a central point of the bottom ofthe cavity.